North Sterling lies on the South Platte Terrace, a component of the Ogallala Formation. Outcrops of Brule Siltstone and interbedded sandstone are present within the park, as are fossils of mostly Cretaceous origin.
The lake sits at an elevation ranging from 4,065 to 4,138 feet on an area known as the South Platte Terrace, a dissected plain covered by thin silty loess deposits, thick eolian sand deposits, and alluvial materials.
The intermittent Cedar and Darby creek drainages cut through the Verdos Alluvium and White River/Chadron Formations in the Stoneham-Cushman-Shingle unit, which underlie much of the reservoir. These soils are subject to soil blowing and are vulnerable to both wind and water erosion.
Probable Rhinocerotidae and Brontotheriidae (ancient rhino ancestors) and other numerous fossils erode out of beds of decomposed Brule siltstone, sandstone, and shale overlay bedrock formations consisting of gneiss, granite, quartz, chert and pegmatite
The fossil jawbone of a Brontothere found on the park can be viewed in a large boulder at the Balanced Rock trailhead near an interpretive sign.
For more information, check the Colorado Geological Survey.